Calculators
Wire Size Calculator
Find the smallest wire gauge that meets ampacity and voltage drop limits for a circuit's current, length, voltage and material.
- Runs in your browser
- Nothing uploaded
The tool
Enter your numbers below. Results update as you type, and nothing leaves your browser.
Everything you enter stays on your own machine. Nothing is sent to us or to anyone else, which you can confirm in your browser network tab.
What people use it for
- Sizing a sub-panel feeder
- Running power to an outbuilding
- Checking voltage drop on long runs
- EV charger circuit planning
About this tool
What the Wire Size Calculator does
Wire must carry the current without overheating (ampacity) and without losing too much voltage over its length. Long runs often need a larger wire than ampacity alone suggests. This calculator checks both: ampacity from the common 75°C insulation column and voltage drop from conductor resistance.
It also works as a wire gauge ampacity calculator or a voltage drop calculator.
Formula
- Voltage drop (single phase) = 2 × K × I × L ÷ circular mils
- Three phase uses √3 instead of 2
- K ≈ 12.9 for copper and 21.2 for aluminium
- Ampacity from the 75°C column of standard conductor tables
Worked example
Using the starting values in the calculator above:
| Input | Value |
|---|---|
| Load current | 40 A |
| One-way circuit length | 75 ft |
| Voltage | 240 |
| Phase | Single phase |
| Conductor | Copper |
| Maximum voltage drop | 3% |
| Result | Value |
|---|---|
| Recommended wire size | 8 AWG |
| Size set by | Ampacity |
| Voltage drop at that size | 1.95% |
| Voltage drop | 4.69 |
| Smallest size by ampacity alone | 8 AWG |
How to read the result
Breaker size, terminal temperature ratings, conduit fill, ambient temperature and continuous-load rules can all require a larger wire than this result.
Small conductors have their own overcurrent limits in the NEC (for example 14 AWG copper on 15 A, 12 AWG on 20 A).
For planning only. Electrical, gas and refrigerant work must follow current codes and be carried out or checked by a licensed professional.
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Questions
Questions about this tool
The things clients actually want to know — costs, timelines, ownership and what happens after launch.
Copper 8 AWG is commonly used at 40 A on short runs with 75°C terminations, but long runs may need 6 AWG to limit voltage drop.
A common recommendation is 3% for a branch circuit and 5% total including the feeder.
It has higher resistance and lower ampacity than copper of the same gauge.
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